Hi Joe,
Thanks for the input, it explains a lot, much appreciated.
Mine's a late 280SL, according to the data card she's a Feb. '71. It had the transistorised ignition, which I removed when it got fried. I've replaced that with a 123 ignition and the hotter Bosch coil. I've noticed that the garage that installed the 123 ignition bypassed the ballast resistors.
Since I've been using the hotter NGK BP5ES plugs I've also found that I have no sooting or carbon on the plugs, they're burning a nice clean rusty grey colour.
The bottom line I guess is that the 5k ohm resistor ends are fine for this set up.
Thanks & best,
Mike